This application is based on and incorporates herein by reference Japanese Patent Application No. 2000-333292 filed on Oct. 31, 2000.
1. Field of the Invention
The present invention relates to an adaptive receiver used in multi-carrier transmission systems for transmitting data in parallel through frequency division.
2. Description of Related Art
In recent years, attention is paid to an OFDM (Orthogonal Frequency Division Multiplexing) system for transmitting data in parallel through frequency division as a modulation system used for broad band communication using radio wave. According to this OFDM system, data can be transmitted using a longer period, and therefore influence of frequency selective fading is reduced. Moreover, in this OFDM system, inter-symbol interference can be prevented even if a delayed radio wave is received by adding a copy signal of the own symbol called as a guard interval.
Even when this OFDM system is used, the feature is lessened to a large extent because the orthogonal property is lessened in the environment where a long-delayed signal which appears after a long period of delay is received after a plurality times of reflection at walls or in the case where the preceding signal wave exists because of synchronization with the delayed wave. The orthogonal property is lessened as shown in FIG. 18A to FIG. 18C.
In FIG. 18A, the delayed wave is within a guard interval. In this case, the orthogonal property is never destroyed because a part (indicated as A in the figure) generating interference with other symbol is cut off with elimination of the guard interval at the time of demodulation. In FIG. 18B, a signal wave exceeding the guard interval appears. In this case, since the part generating interference with the other symbol enters a FFT window, the orthogonal property of carrier is destroyed at the time of demodulation. In FIG. 18C, synchronization is set up with the delayed signal wave. Similarly to the case of FIG. 18B, since the part generating interference with the other symbol enters the FFT window, the orthogonal property of carrier is destroyed at the time of demodulation.
An adaptive array antenna is proposed to effectively control an unwanted signal wave (such as delayed wave exceeding the guard interval, preceding wave or interference wave) having a lower correlation to such a desired signal wave. Here, the adaptive array antenna is a device comprising an array antenna made up of a plurality of antenna elements and an adaptive arithmetic unit for calculating a complex weight for changing amplitude and phase of an output signal of each antenna element in view of combining the signal waves by multiplying the receiving signals of antenna with a complex weight to extract the desired signal wave and suppress the unwanted wave.
Moreover, the arithmetic operation by the adaptive arithmetic unit is called as an adaptive algorithm, several algorithms have been reported. For example, algorithms LMS (Least Means Square) and CMA (Constant Modulus Algorithm) can be listed as those based on an MMSE (Minimum Mean Square Error). The LMS is the algorithm for calculating the optimum complex weight by minimizing a difference between a reference signal as a desired array response and an actual array output signal. Moreover, the CMA is the algorithm that can be applied to a constant envelope signal to calculate an optimum complex weight to recover a signal distorted in a multipath environment to an original constant envelope signal. Moreover, the additional algorithms are MSN (Maximum Signal to Noise Ratio) for calculating the complex weight to obtain a maximum SNR (Signal to Noise Ratio) of array output and DCMP (Directionally Constrained Minimization of Power) for minimizing the power under the precondition that the incoming direction of the desired signal wave is known.
Moreover, JP-A-10-93323 (U.S. Pat. No. 6,087,986) teaches an adaptive antenna and multi-carrier radio communication system. This system conducts an adaptive arithmetic operation based on LMS, RLS (Recursive Least Squares) algorithm for extracting the known signal called as pilot data inserted into the receiving signal and then updating the complex weight to minimize an error from the known signal prepared in the receiver side. JP-A-10-210099 also teaches an adaptive receiver which realizes CMA in a frequency range for updating the complex weight to make constant the spectrum after the adaptive combining.
The unwanted wave having lower correlation can be controlled effectively by utilizing the above adaptive array antenna. However, the unwanted wave (delayed signal wave within the guard interval) having higher correlation cannot be controlled effectively and thereby the demodulation characteristic is not satisfactory. This is because, a propagation path in the adaptive array antenna is estimated by comparing two reference signals, one resulting after combining the receiving signal weighted by the adaptive array antenna and the other being a stored known signal, and hence the weighting by the adaptive array antenna influences on the propagation path estimation value.
It is therefore an object of the present invention to provide an adaptive receiver which can effectively control not only an unwanted signal wave of lower correlation but also an unwanted signal wave of higher correlation.
It is another object of the present invention to remove influence of weighting in an adaptive array antenna on estimation of a propagation path estimation value.
According to the present invention, an adaptive receiver includes an adaptive array unit and an equalizer unit. In the adaptive array unit, receiving signals of an array antenna are weighted by multiplying with a complex weight calculated by an adaptive arithmetic unit, and adaptively combined. In the equalizer unit, propagation path estimation values are calculated from the receiving signals by a propagation path estimating unit, weighted with the complex weight by a complex weight adjusting unit, and adaptively combined. An output signal of the adaptive receiver is produced based on the combined signal and the combined propagation path estimation value.